As is known, lithography is commonly used when manufacturing integrated circuits. The process generally includes forming a photoresist layer on the surface of a semiconductor wafer, and then positioning a mask over the resist-coated wafer. The mask typically has light non-transmissive (opaque) regions of chrome and light transmissive (transparent) regions of quartz. Radiation from a light source (e.g., ultra-violet or deep ultra-violet light, etc) and focused via an optical lens system is then applied to the mask. The light passes through the transparent mask regions and exposes the underlying photoresist layer, and is blocked by the opaque mask regions to leave those underlying portions of photoresist layer unexposed. Depending on the specific process used, either the exposed or non-exposed regions of photoresist layer can then be removed, thereby leaving a patterned resist layer on the wafer, which in turn allows for subsequent processing of the wafer such as, for example, etching, depositing, and other typical semiconductor processes.
One conventional technique that enables smaller minimum device dimensions with relatively good contrast is generally referred to as alternating phase shift masking (APSM). APSM exploits the destructive interference caused by light passing through two adjacent transparent mask areas to create an unexposed area on the photoresist layer. In short, the phase of the light wave passing through and exiting the transparent mask regions is a function of the mask thickness, so mask thickness of adjacent mask regions can be selectively varied, such that the light exiting from those adjacent regions is out of phase which causes destructive interference (e.g., where light waves exiting from one adjacent area have a phase that is out-of-phase with the phase of light waves exiting from the other adjacent area, and the waves therefore cancel one another or otherwise yield a combined wave of diminished amplitude). Since the photoresist material is responsive to the intensity of the light, an unexposed area will be formed on the photoresist layer where the transparent regions of differing thicknesses are adjacent.